1. Field of Invention
This invention relates to a combined magnetic/centrifugal-fluid seal for hermetically sealing a rotatable shaft supported in a closely-fitted housing in a non-contacting, non-wearing manner.
More particularly, the invention relates to a hermetic seal that comprises a combined magnetic seal employing a ferrofluid for hermetically sealing a rotatable shaft in the zero and low speed range and which utilizes the ferrofluid to form a centrifugal fluid seal at higher shaft speeds.
2. Background Problem
There are many applications, such as in pumps, compressors and the like, where hermetic sealing of a rotatable shaft in a non-contacting, non-wearing manner is not only desirable but essential during both zero and low speed operation and during high speed rotation. Because of the high speed sealing requirement for such applications, and in order to reduce wear, it is desirable that the seal be non-contacting in nature.
One well known form of non-contacting seal for use at zero and low speeds is the magnetic ferrofluid seals of the type described in U.S. Pat. No. 2,863,538--issued Dec. 9, 1958 for a "Permanent Magnet Seal"; U.S. Pat. No. 3,746,407--issued July 17, 1973 for a "Ferro Hydrodynamic Low Friction Bearing"; and an article entitled "Magnetic-Fluid Seal" appearing in Machine Design magazine, Mar. 28, 1968 issue, pages 145-150, for example. While known designs of ferrofluid seals can operate satisfactorily at zero speed and low speeds, as in a vacuum chamber feed-through, they tend to break down and leak at high rotational speeds.
A known form of non-contacting seal which can operate satisfactorily at high rotational speeds is the centrifugal seal of the type described in U.S. Pat. No. 3,694,042--issued Sept. 26, 1972 for a "Radial Fluid Bearing", for example. Such known centrifugal seals can support a large pressure difference based on the level difference of a pool of fluid formed on the two sides of a rotating disk attached to a shaft. The pool of fluid is both formed and maintained by centrifugal force effects. However, centrifugal seals have little capacity at low rotational speeds and collapse and leak at standstill.
In order to overcome certain of the above briefly-discussed problems in a non-contacting seal configuration, one proposed solution is described in U.S. Pat. No. 3,097,853--issued July 16, 1963 for "Dynamic Seal". This patent describes a seal configuration which employs an electrically conductive fluid in an otherwise conventional centrifugal seal together with an electromagnet for pumping the electrically conductive liquid during low speed and zero speed in order to maintain the hermetic sealing capabilities of the structure. The arrangement, however, has many disadvantages not the least of which is its relative complexity, power consumption at the zero or low speed regions and use of mercury as a sealing liquid.